Electronic gage amplifiers are commonly used in industry in combination with electronic gages (i.e., transducers) to measure precision crafted components. Uses of such devices are practically endless. Such electronic gage amplifiers interface to one or more transducer gages of a type, for example, that convert a distance measurement into an electrical signal. The gages used for such applications are typically either linear variable differential transformer (L.V.D.T.) type transducers or half-bridge transducers. The invention, however, may be used with various types of transducers, including transducers which measure variables other than distance.
An electronic gage amplifier will typically have a plurality of electrical jacks for connecting gages to the amplifier. For either L.V.D.T. or half bridge transducers, the qage amplifier provides two sinusoidal driving voltages to one or two coils within the body of each gage. The voltages are of equal amplitude and frequency but opposite phase (i.e. 180.degree. offset from each other). These driving voltages will hereinafter be referred to as SINE and SINE. The gage provides one or two output line(s) back to the qage amplifier which carry some combination of the SINE and SINE driving signals, the combination depending on the position of the gage head. The gage head is the portion of the gage which contacts the part being measured and whose displacement in relation to the transducer coils is the distance measured. The gage head in either the L.V.D.T. or half-bridge type gage has a point in the center of the range of displacement of the gage head defined as the zero point, at which the driving signals combine to cancel each other so that the signal on the output line is zero volts. When the gage head is displaced to one side of the zero point, a sinusoidal voltage having frequency equal to the driving voltages and phase equal to that of one of the driving voltages appears on the output line(s). The amplitude is directly related to the distance of displacement from the zero point. When the qage head is displaced from the zero point in the opposite direction, the signal on the output line(s) is a sinusoidal voltage with a phase equal to that of the other driving voltage. As before, the amplitude is directly related to the displacement from the zero point.
FIG. 1 shows a block diagram of the circuitry of a typical electronic gage amplifier unit of the prior art. This particular embodiment is designed for use with two gages, designated the "A" gage and the "B" gage, respectively. The unit circuitry consists of an oscillator 12, an AC amplifier 14, a synchronous passive demodulator 16, a DC amplifier 18 and a display 20. The oscillator 12 provides to the gages SINE and SINE signals having a frequency typically in the range of 5 to 15 kHz, on lines 13A and 13B, respectively. The SINE and SINE outputs, which differ in phase by 180.degree. , are also sent to the demodulator 16. The amplifier 14 receives a sinusoidal response signal from each of gages A and B, such sinusoidal signals having phase and amplitude indicative of a distance measurement. The amplifier 14 is a sine-wave amplifier and has a voltage gain on the order of 1000. The output of the amplifier 14 is fed into demodulator 16. Demodulator 16 is a synchronous passive demodulator which provides a steady-state voltage in response to the input from the carrier amplifier 14. The DC signal output of demodulator 16 is then fed into signal amplifier 18, where it is further amplified and sent to the display 20. Typically, the display 20 is either an analog display having a magnetic needle pointer or a digital display.
Two types of errors are typically introduced to the measurement reading by the electronic gage amplifier circuitry. The first type of error is termed the zero, or offset, error. This type of error causes the output on line 19 (see FIG. 1) to be slightly offset from the value that it should ideally provide in response to the signal from the gage head. This is caused by fixed D.C. offset voltages and thermal drift of the electronics which is inherent to the circuitry of the unit. The second type of error is termed span, or sensitivity, error and is an error that increases in value as the measurement reading gets further away from the center point of the span error. Due to aging of components and temperature dependent circuitry operation, the amplification in the circuitry will vary slightly from the desired amplification. For instance, if the carrier amplifier 14 is nominally designed to provide a gain of 1000, it may in fact be providing a gain of only 999.
Additionally, it is inherent to all gages of the L.V.D.T. or half-bridge types that they supply, in addition to the measurement signal, a quadrature voltage. The latter is an error voltage which effects the accuracy of the measurement as read by the gage amplifier. This is not an error produced by the circuitry of the electronic gage amplifier but, rather, by the gages themselves. This quadrature error voltage is a sinusoidal voltage which is 90.degree. out of phase with both the SINE and SINE driving voltages as well as with the gage s own valid output voltage. The quadrature error voltage is due to impedance mismatch between the coils in the gage. Unfortunately, it is virtually impossible to provide perfectly matched coils. When measuring small dimensions, the amplitude of the quadrature voltage is relatively large in comparison with the valid measurement signal. In the electronic gage amplifiers of the prior art as shown in FIG. 1, the quadrature voltage is amplified along with the signal voltage. This high quadrature voltage from a gage head can saturate the amplifier 14 and result in inaccurate operation and significant errors in the measurement readings.
Another error which is typical of certain gage heads is a phase shift error. Instead of the sinusoidal output of the gage being exactly in phase with the SINE or SINE driving voltage, the output of the gage head is typically phase shifted approximately 5.degree. to 10.degree. from the driving voltage. This phase shift is a constant for any given gage. Prior art electronic gage amplifiers typically correct for this phenomenon by inserting external phase correction circuitry between the electronic gage amplifier and the gages to equalize the qage output phase to the driving voltage phase.
The operating controls for the gage amplifier are shown generally at 22. A group of knobs, switches and/or buttons is typically provided on the surface of the housing of the gage amplifier for controlling certain operating functions. The three most essential controls are shown at 22. The range control 24 is used to set the gage amplifier to one of a specific group of full scale ranges. For instance, the Model TA Comparator Gage Amplifier of Taper Micrometer Corporation of Worcester, Massachusetts provides two possible full scale ranges, +/-0.001" and +/-0.0001". Other prior art gage amplifiers have up to five or more possible range settings. Typically, the range control switch 24 operates by switching in different resistance values across an operational amplifier within amplifier block 14 to adjust its gain and provide the different ranges. Resistor switching, however, as is true with any devices for altering hardware within a system, creates reliability problems for the unit and introduces additional errors to the measurement readings because of the thermal characteristics of switches as well as other reasons.
Zero control 26 is used to adjust the point in the travel of the qage head at which a zero will be displayed on the amplifier display 20. In most cases, it would be desirable to set the display zero at the mechanical zero point of the gage head, which is at the exact center of the gage head range of displacement, because the qage operates most linearly, and therefore most accurately, around the mechanical zero point. Occasionally, however, it is convenient to set the display zero at a different point. Prior art electronic gage amplifiers typically provide a potentiometer for adjusting the zero point. The unit may have a mark indicating where the potentiometer must be positioned to match the display zero to the mechanical zero. Manual potentiometer adjustments, however, are far from accurate.
The function control switch 28 can be of a variety of forms. Basically, function control switch 28 is used to instruct the electronic gage amplifier what information to read and display. In its most basic form it is used to choose between displaying what is read on gage A and what is read on gage B. In more advanced forms it can be used to effect display of the difference between gage A or qage B, the summation of gage A and gage B, the average of gage A or gage B and many other possibilities.
In the electronic qage amplifiers of the prior art, particularly the analog magnetic needle type, the display will not show the actual absolute value of the measurement, but rather, a scaled value. The operator must look at both the displayed value and the range setting control 24 in order to determine what measurement he is reading. Not only is it inconvenient for the operator to look to two places, but it also frequently leads to errors because the operator must perform mental arithmetic while reading the data and further because the range setting control 24 can be easily misread.
Periodic calibration of numerous parameters is necessary in order to keep the gage amplifier working accurately. The calibration controls are usually provided within the enclosure of the electronic gage amplifier since they should only be adjusted by a skilled technician and not by the average user. Periodic calibration is necessary to correct for aging and electronic drift of components. Gain control 30 and 32, for gages A and B, respectively, for sine-wave amplifier 14 must be adjusted to match amplifier sensitivity (or span) to each gage head. These controls are subject to damage as they are readjusted each time a new gage head is used. There is also an offset control 34 for the demodulator and a second offset control 36 for the DC amplifier 18, to correct for electronic drift of the zero point of the gage amplifier. Another gain control 38 may be provided for the DC amplifier 18.
There are numerous shortcomings to the prior art electronic gage amplifiers. For instance, every time the range setting 24 is changed, the zero control 26 must be readjusted because each time a resistor is switched across the operational amplifier in block 14, new offset errors and phase errors are introduced in the circuitry. Changing ranges after the initial set up of a qage head also causes sensitivity errors to occur in the measurement. Further, if a measurement is off scale in one range, it is necessary for the operator to manually change range control 24 to select a different scale.
Additionally, the prior art electronic gage amplifiers do not provide an accurate means for locating the mechanical zero point of the gage head. As stated earlier, it is desirable to center the measurement range around the mechanical zero point of the gage head since it is the most linear operating region.
Therefore, it is an object of the present invention to provide an improved electronic qage amplifier.
It is a further object of the present invention to provide an electronic gage amplifier which eliminates the need for periodic calibration.
It is yet another object of the present invention to provide an electronic gage amplifier wherein all relevant information about the measurement is provided to the operator in one display location, at a single glance.
It is a further object of the present invention to provides an electronic qage amplifier with means for quickly locating the exact mechanical zero point of the gage head.
It is yet another object of the present invention to provide an amplifier which offers hands off operation by automatically changing ranges
It is a further object of the present invention to provide an amplifier which introduces no gain or offset error when the range settings are changed.